The quality of a television picture depends upon the reception of an input signal from the antenna having a value within predetermined limits. Television receivers are normally designed with automatic gain control circuits such as taught by Leland in U.S. Pat. No. 3,622,891 which detune one or more of the input stages to keep the subsequent amplification stages of the receiver from saturating in the presence of a strong input signal. Alternately, as taught by Wilcox in U.S. Pat. No. 3,619,786, the receiver may include one or more attenuators which attenuate the received signal in response to an automatic gain control signal. The function of these circuits, as well as others known in the art, is to maintain the received television signal within the predetermined limits.
As the distance between the transmitter and receiver increases, the signals received by the receiver become weaker. High gain television antennas have been developed to extend the distance over which a satisfactory picture can be obtained. Unfortunately, these high gain antennas are highly directional and the antenna must be pointed in the direction of the transmitter to achieve the desired gain. To overcome this problem, rotor mechanisms, such as disclosed by Jordan in U.S. Pat. No. 2,498,957 have been developed which permit the operator to point the high gain antenna in the direction of the transmitter.
This combination works well when the transmitter is located relatively far from the receiver; however, when a transmitter is relatively close, the received signal may exceed the predetermined limits of the television receiver and cause the input stage of the receiver to be overpowerd distorting the received signal and, therefore, the picture. This problem can often be overcome by rotating the antenna away from the transmitter, decreasing the gain of the antenna, or by attenuating the received signal as taught by Cuvilliez in U.S. Pat. No. 2,654,030. Rotating the antenna away from the transmitter is generally unsatisfactory since more often than not, it results in ghost images produced by signals reflected from objects located along the path in which the antenna is pointed. Alternately, Cuvilliez teaches a five position switch to control the strength of the received signal at the input of the receiver. In the first position, the signal from the antenna is applied to the input of the receiver in a normal manner. The second position merely reverses the input leads from the antenna to the receiver. The third position inserts an impedance in series with the antenna leads to attenuate the signal strength. The fourth position disconnects the antenna from the receiver so that the signal is received only by the lead wires connecting the control to the receiver. The fifth position grounds the leads at the controller so that the received signal is a ground signal. This permits the operator to compensate for the strength of the received signal by switching the control from one position to another.
Preamplifiers have been developed to increase the reception range of receivers over that normally obtained by a high gain antenna. The preamplifier raises the signal level of very weak signals to within the predetermined limits of the receiver. This technique works well when receiving signals from distant stations but suffers a major disadvantage when the receiver antenna is close to the transmitter. In this instance, the preamplified signal is too strong and the subsequent amplification stages of the receiver are overdriven, thereby causing a "grassy" or discolored picture.
For this reason, some efforts have been made at utilizing two antennas for a single television receiver. In such a system, one large antenna is used to receive signals from the distant transmitting stations and a different smaller antenna is used to receive signals from the closer television stations. Then according to the distance of the television transmitter from which reception is desired, the operator selects one of the two antennas by way of a switch located in the proximity of the television set. Such a system offers some improvement over a single antenna combined with a preamplification system but suffers a substantial disadvantage in that frequently the two antennas will reflect signals to one another so that an optimum picture is not received while either antenna is in use.